This invention relates to dynamoelectric machines and in particular to the rotors of such machines which employ rotor windings having at least two adjacent winding laminations electrically connected in parallel.
In a large dynamoelectric generator, a rotor having field windings thereon is rotated within a fixed stator assembly. The rotor windings are configured so as to define a plurality of pole faces. Typically, this winding on the rotor is electrically energized through collector rings surrounding the rotor shaft. Electrical current flowing through the rotor windings produces a rotating magnetic field which, in turn, produces an electrical current in conductive windings in the stator portion of the machine during rotation of the rotor assembly. Such rotors generally operate at a rotational speed of either 1,800 or 3,600 rpm although other speeds are common outside the U.S. In large dynamoelectric machines, the electrical power rating is generally measured in hundreds of megawatts. Accordingly, there are significant cooling requirements associated with such machines. For example, the stator portion of a large generator is always cooled using water or some other liquid. However, cooling of the rotor is generally accomplished by employing an atmosphere of hydrogen gas. However, the cooling path actually followed by the hydrogen gas is generally dependent on other machine design features. Such features particularly include the number of machine poles. For example, a four-pole machine is typically cooled by means of radially directed channels through the conductive windings. On the other hand, two-pole machines are typically cooled by means of diagonally directed channels, such as those shown in U.S. Pat. Nos. 3,995,180 issued Nov. 30, 1976 to Walter B. Giles and 3,348,081 issued Oct. 17, 1967 to David M. Willyoung, both of which are assigned to the same assignee as the present invention. These patents are hereby incorporated herein by reference as background material for the present invention.
The rotor essentially comprises a substantially cylindrical rotor core with longitudinal slots along its outer circumference into which the rotor field windings are placed. There is typically one rotor winding for each pole in the machine. Each pole winding is disposed in several slots on either side of the pole face which is part of the rotor core. Each pole winding comprises conductors which are laid into the pair of slots closest to the pole faces at the bottoms thereof and subsequent winding layers are added to the pair of slots in the form of a helix until a position near the top of the slots is reached, after which the winding is continued in the next pair of rotor slots. In a similar fashion, these windings are arranged in a helix which descends one layer at a time until the bottoms of the second slot pair are reached. This winding scheme is repeated for the desired number of slot pairs. Likewise, the flow of current through these windings follows the conductive path just described. The conductors are typically flat copper bars with holes along the lengths thereof to accommodate the cooling method employed in each particular case. The holes in each bar are aligned with holes in adjacent bars to form the appropriate cooling channel. At the end portions of the windings, the conductive bars that run the length of the slot are connected by means of copper end sections to corresponding conductive bars located in a slot symmetrically disposed with respect to the pole face. Because the winding material is conductive, the series wound nature of the rotor pole windings must be preserved by disposing layers of insulation between adjacent copper layers in the slots. Furthermore, this insulation must also extend into the end winding regions of the rotor to keep the winding configured in a series connection. Thus, heretofore, each pole winding generally comprised a single series electrical circuit.
As noted above, cooling of the rotor is essential for large dynamoelectric machines. The aforementioned Willyoung patent describes one method of cooling the copper conductive winding of a two-pole machine, particularly in and along the longitudinal center of the rotor. However, cooling of the end winding regions is also important if proper operating characteristics are to be maintained. One method of providing cooling to the end regions of the rotor winding is to employ parallel-connected copper bars in the rotor winding slots. Furthermore, fabrication of the rotor windings and subsequent assembly of the windings into the rotor slots is facilitated by the use of electrically parallel but mechanically separate rotor slot bars. For example, a rotor winding bar that was X inches thick would be replaced by two rotor winding bars, each of which were X/2 inches thick. These bars are connected in parallel through a brazing process employed at the end of each slot. Additionally, near the ends of the winding bars there may be grooves in each bar which form a common channel when the rotor bar halves are fitted together. Since such bars are connected in parallel there is no need for insulating material between them.
However, it has recently been noted that if a dynamoelectric machine is so constructed, a problem of copper galling may exist. That is to say, certain accumulations of copper particles may occur because of mechanical abrasion between these parallel-connected copper laminations. For clarity, and ease of understanding, the term layer, as used herein and in the appended claims, refers to a series-connected pole winding portion; furthermore, the term lamination, refers to adjacent parallel-connected conductive winding portions; thus a layer comprises one or more laminations. It is these laminations which may contain longitudinally extending cooling channels.
During normal machine running at relatively high speeds, there does not appear to be a significant amount of movement between adjacent laminations. This is thought to be due to the large centrifugal forces which occur and which hold the rotor bars in a substantially fixed position. However, during certain times, the machine is not generating power and a small angular velocity of rotor motion is maintained by running the machine through a turning gear so as to prevent any bowing or deformation of the rotor forging. It is thought that this low-speed mode of operation may contribute to relative motion between adjacent parallel-connected laminations (not to be confused with the insulated series-connected layers). Since these laminations are at the same voltage along their lengths, there is no need to provide electrical insulation between them. Thus, they are in mechanical contact along the length of rotor slot. However, the relative motion between laminations may produce galling of the copper conductive material. The galling may result in an accumulation of copper particles which could eventually lead to electrical grounding problems in the rotor winding circuit. This galling is an unexpected occurrence. Nonetheless, it is still desirable to connect certain rotor winding bars in parallel for purposes of end winding cooling and also for ease of assembly.